Fiber-coupled laser with time varying beam characteristics

ABSTRACT

Methods, apparatus, and systems comprising a fiber-coupled laser and time-varying beam characteristics. A laser may generate an optical beam that is launched into one or more lengths of fiber, at least one of which comprises a confinement region that is optically coupled to an output. A perturbation device may modulate, through action upon the one or more lengths of fiber, a beam characteristic over a time period during which the laser is energized. A controller may cause the perturbation device to act upon the one or more lengths of fiber to impart a time-averaged beam characteristic and/or to induce a continuous variation in one or more beam characteristics during system use. A process monitor may sense a metric external to the optical system, and a feedback signal from the process monitor may be coupled into the controller. Dynamic beam characteristics may be modulated based on the feedback signal.

RELATED APPLICATIONS

This application is a continuation in part (CIP) of, and claims the benefit of, U.S. patent applications Ser. Nos. 15/607,399, 15/607,410 and 15/607,411, all of which have the title “ADJUSTABLE BEAM CHARACTERISTICS”, and all of which were filed on May 26, 2017, and all of which further claim priority to U.S. Provisional Application No. 62/401,650, filed on Sep. 29, 2016, entitled “ALL-FIBER DEVICE FOR VARYING THE BEAM QUALITY OF A LASER”. These priority applications are incorporated by reference herein in their entireties for all purposes. This application further claims priority to PCT Application PCT/US17/34848, having the title “ADJUSTABLE BEAM CHARACTERISTICS”, and also filed on May 26, 2017, which claims priority to U.S. Provisional Application No. 62/401,650, filed on Sep. 29, 2016, entitled “ALL-FIBER DEVICE FOR VARYING THE BEAM QUALITY OF A LASER”. The international application is also incorporated by reference herein in its entirety for all purposes.

TECHNICAL FIELD

The technology disclosed herein relates to fiber-coupled laser systems. More particularly, the disclosed technology relates to fiber-based optical beam modulation methods, apparatus, and systems.

BACKGROUND

Optical systems have many applications, including materials processing, sensing, illumination, and medical applications. Such optical systems often employ lasers, for example, fiber lasers, disk lasers, diode lasers, diode-pumped solid state lasers, and lamp-pumped solid state lasers. In these systems, optical power is often delivered by an optical fiber.

In some applications, it may be useful to vary optical beam characteristics (intensity distribution, divergence distribution, etc.), for example to optimize a process. Techniques to vary optical beam characteristics over time may facilitate realization of optimum beam characteristics, may facilitate rapid averaging or sweeping of the beam characteristics to provide beam smoothing via spatial averaging, and/or may result in speckle reduction. The optimum rate and amplitude of modulation, as well as the desired instantaneous and average beam characteristics, depend on the application. Most conventional lasers and optical systems including conventional lasers do not allow variation of a beam characteristic, particularly on short timescales. Free-space optics downstream from the laser could potentially provide some beam tuning capabilities, but such an approach entails significant cost, complexity, reduced reliability (sensitivity to contamination or misalignment of the optics), optical loss, and/or practical limitations (e.g., increased size and weight of process head, environmental sensitivity).

A fiber-based, or “all-fiber,” system for varying beam characteristics overcoming the limitations listed above would therefore be advantageous by eliminating one or more of the constraints associated with free-space optics.

SUMMARY

Disclosed herein are at least fiber-based methods, systems, and apparatus for varying characteristics an optical beam. Methods may include receiving an optical beam into one or more lengths of fiber, where at least one of the lengths of fiber comprises a confinement region that is optically coupled to an optical output. One or more beam characteristics of the optical beam are varied through action upon the one or more lengths of fiber over a period of time of the receiving. Methods may further include energizing a laser to generate the optical beam, and varying the one or more beam characteristics while the laser is energized. Hence, action upon the one or more lengths of fiber may occur during a setup phase to establish an initial operating point for an optical delivery system, and action upon the one or more lengths of fiber may further occur during a run-time phase to time average one or more beam characteristics, such as intensity (spatial) distribution. In some examples, methods include varying the one or more beam characteristics continuously about a beam characteristic target. The target may be updated, for example based on a feedback signal that may be derived from process metric data. Process metric data may, for example, be collected with any process monitor suitable for detecting a response that is indicative of a process metric. Exemplary process monitors include a microphone, a spectrometer, a pyrometer, an optical sensor, or any combination thereof. Methods may include varying one or more beam characteristics based on the feedback signal. One or more beam characteristics may be continuously varied according to a sinusoidal modulation signal, or other periodic modulation signal, having a predetermined amplitude and a predetermined frequency. A beam characteristic target may be moved or updated by determining a correlation between the feedback signal and the modulation signal. The feedback signal may be compared with the modulation signal, for example with a lock-in detection algorithm, to determine a correlation between the feedback signal and the modulation signal. In response to detecting a process drift, for example as a result of workpiece variations, the beam characteristic target may then be updated (sometimes known as “closed-loop control”).

Systems disclosed herein may include an optical beam delivery system comprising an optical fiber. The optical system may include one or more lengths of fiber and may be devoid of free-space optics. The optical system may include a laser. The laser may comprise any suitable means to energize an optical gain medium. In some examples, the gain medium comprises fiber, and the energizing means comprises a pump laser. The optical system may further include a perturbation device and a controller coupled to the perturbation device. The controller may control the perturbation device to vary the one or more beam characteristics over a period of time that the laser is energized. The one or more beam characteristics may be, for example, beam diameter, divergence distribution, beam parameter product, intensity distribution, luminance, optical intensity, power density, radiance, or radial beam position. The perturbation device may act upon the one or more lengths of fiber to alter a coupling of the optical beam between a first length of the fiber and the confinement region of a second length of the fiber or between confinement regions within a single length of fiber.

In some examples, the controller is to vary the one or more beam characteristics according to a modulation signal having a predetermined shape, a predetermined amplitude and/or a predetermined frequency. The optical system may further include a process monitor to output a feedback signal to the controller. The process monitor may, for example, include at least one of an acoustic sensor or an optical sensor. The process monitor is to detect a response external of the delivery device that is indicative of a process metric. The controller may then vary the one or more beam characteristics based on the feedback signal. For examples where the one or more beam characteristics are varied according to a sinusoidal modulation signal having a predetermined amplitude and a predetermined frequency, a lock-in amplifier may compare the feedback signal with the modulation signal. With a correlation between the feedback signal and the modulation signal, a target value of one or more beam characteristics may be updated in response to any process metric drift during operation of the optical system.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, wherein like reference numerals represent like elements, are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the presently disclosed technology. In the drawings,

FIG. 1 illustrates an example fiber structure for providing a laser beam having variable beam characteristics;

FIG. 2 depicts a cross-sectional view of an example fiber structure for delivering a beam with variable beam characteristics;

FIG. 3 illustrates an example method of perturbing a fiber structure for providing a beam having variable beam characteristics;

FIG. 4 is a graph illustrating the calculated spatial profile of the lowest-order mode (LP01) for a first length of a fiber for different fiber bend radii;

FIG. 5 illustrates an example of a two-dimensional intensity distribution at a junction when a fiber for varying beam characteristics is nearly straight;

FIG. 6 illustrates an example of a two-dimensional intensity distribution at a junction when a fiber for varying beam characteristics is bent with a radius chosen to preferentially excite a particular confinement region of a second length of fiber;

FIGS. 7-10 depict experimental results to illustrate further output beams for various bend radii of a fiber for varying beam characteristics shown in FIG. 2;

FIGS. 11-16 illustrate cross-sectional views of example first lengths of fiber for enabling adjustment of beam characteristics in a fiber assembly;

FIGS. 17-19 illustrate cross-sectional views of example second lengths of fiber (“confinement fibers”) for confining adjusted beam characteristics in a fiber assembly;

FIGS. 20 and 21 illustrate cross-sectional views of example second lengths of fiber for changing a divergence angle of and confining an adjusted beam in a fiber assembly configured to provide variable beam characteristics;

FIG. 22A illustrates an example laser system including a fiber assembly configured to provide variable beam characteristics disposed between a feeding fiber and process head;

FIG. 22B illustrates an example a laser system including a fiber assembly configured to provide variable beam characteristics disposed between a feeding fiber and process head;

FIG. 23 illustrates an example laser system including a fiber assembly configured to provide variable beam characteristics disposed between a feeding fiber and multiple process fibers;

FIG. 24 illustrates examples of various perturbation assemblies for providing variable beam characteristics according to various examples provided herein;

FIG. 25 illustrates an example process for adjusting and maintaining modified characteristics of an optical beam;

FIGS. 26-28 are cross-sectional views illustrating example second lengths of fiber (“confinement fibers”) for confining adjusted beam characteristics in a fiber assembly.

FIG. 29 illustrates example methods of varying optical beam characteristics;

FIG. 30 illustrates a schematic of an optical beam delivery system, in accordance with some embodiments;

FIG. 31 is a timing diagram illustrating operation of an optical beam delivery system having variable optical beam characteristics in accordance with some embodiments;

FIG. 32 illustrates a schematic of an optical beam delivery system including a process monitor and feedback signal, in accordance with some embodiments;

FIG. 33 is a timing diagram illustrating operation of an optical beam delivery system having variable optical beam characteristics in accordance with some embodiments;

FIG. 34 illustrates a graph of an exemplary process metric and a variable beam characteristic as a function of process time, in accordance with some embodiments; and

FIG. 35 schematic of an exemplary networked computer system, which may be configured as a controller to read one or more computer readable media and to execute one or more of the methods described herein.

DETAILED DESCRIPTION

As used herein throughout this disclosure and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items. Also, the terms “modify” and “adjust” are used interchangeably to mean “alter.”

The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high- level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.

In some examples, values, procedures, or apparatus are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation.

Reference throughout this specification to “an embodiment” or “one embodiment” or “some embodiments” means that a particular feature, structure, function, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in an embodiment” or “in one embodiment” or “some embodiments” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive.

A “device” may generally refer to an apparatus according to the context of the usage of that term. For example, a device may refer to a stack of layers or structures, a single structure or layer, a connection of various structures having active and/or passive elements, etc. Generally a device is a three dimensional structure with a lateral x-y plane and a height along the z direction within an x-y-z Cartesian coordinate system. The plane of the device may also be the plane of an apparatus, which comprises the device.

Unless otherwise specified the use of the ordinal adjectives “first,” “second,” and “third,” etc., to describe a common object, merely indicates that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner.

As used throughout this description, and in the claims, a list of items joined by the term “at least one of” or “one or more of” can mean any combination of the listed terms. For example, the phrase “at least one of A, B or C” can mean A; B; C; A and B; A and C; B and C; or A, B and C.

Definitions

Definitions of words and terms as used herein:

1. The term “beam characteristics” refers to one or more of the following terms used to describe an optical beam. In general, the beam characteristics of most interest depend on the specifics of the application or optical system. 2. The term “beam diameter” is defined as the distance across the center of the beam along an axis for which the irradiance (intensity) equals 1/e² of the maximum irradiance. While examples disclosed herein generally use beams that propagate in azimuthally symmetric modes, elliptical or other beam shapes can be used, and beam diameter can be different along different axes. Circular beams are characterized by a single beam diameter. Other beam shapes can have different beam diameters along different axes. 3. The term “spot size” is the radial distance (radius) from the center point of maximum irradiance to the 1/e² point. 4. The term “beam divergence distribution” is the power vs. the full cone angle. This quantity is sometimes called the “angular distribution” or “NA distribution.” 5. The term “beam parameter product” (BPP) of a laser beam is defined as the product of the beam radius (measured at the beam waist) and the beam divergence half-angle (measured in the far field). The units of BPP are typically mm-mrad. 6. A “confinement fiber” is defined to be a fiber that possesses one or more confinement regions, wherein a confinement region comprises a higher-index region (core region) surrounded by a lower-index region (cladding region). The RIP of a confinement fiber may include one or more higher-index regions (core regions) surrounded by lower-index regions (cladding regions), wherein light is guided in the higher-index regions. Each confinement region and each cladding region can have any RIP, including but not limited to step-index and graded-index. The confinement regions may or may not be concentric and may be a variety of shapes such as circular, annular, polygonal, arcuate, elliptical, or irregular, or the like or any combination thereof. The confinement regions in a particular confinement fiber may all have the same shape or may be different shapes. Moreover, confinement regions may be co-axial or may have offset axes with respect to one another. Confinement regions may be of uniform thickness about a central axis in the longitudinal direction, or the thicknesses may vary about the central axis in the longitudinal direction. 7. The term “intensity distribution” refers to optical intensity as a function of position along a line (1D profile) or on a plane (2D profile). The line or plane is usually taken perpendicular to the propagation direction of the light. It is a quantitative property. 8. “Luminance” is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. 9. “M² factor” (also called “beam quality factor” or “beam propagation factor”) is a dimensionless parameter for quantifying the beam quality of laser beams, with M²=1 being a diffraction-limited beam, and larger M² values corresponding to lower beam quality. M² is equal to the BPP divided by λ/π, where λ is the wavelength of the beam in microns (if BPP is expressed in units of mm-mrad). 10. The term “numerical aperture” or “NA” of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. 11. The term “optical intensity” is not an official (SI) unit, but is used to denote incident power per unit area on a surface or passing through a plane. 12. The term “power density” refers to optical power per unit area, although this is also referred to as “optical intensity” and “fluence.” 13. The term “radial beam position” refers to the position of a beam in a fiber measured with respect to the center of the fiber core in a direction perpendicular to the fiber axis. 14. The term “radiance” is the radiation emitted per unit solid angle in a given direction by a unit area of an optical source (e.g., a laser). Radiance may be altered by changing the beam intensity distribution and/or beam divergence profile or distribution. The ability to vary the radiance profile of a laser beam implies the ability to vary the BPP. 15. The term “refractive-index profile” or “RIP” refers to the refractive index as a function of position along a line (1D) or in a plane (2D) perpendicular to the fiber axis. Many fibers are azimuthally symmetric, in which case the 1D RIP is identical for any azimuthal angle. 16. A “step-index fiber” has a RIP that is flat (refractive index independent of position) within the fiber core. 17. A “graded-index fiber” has a RIP in which the refractive index decreases with increasing radial position (i.e., with increasing distance from the center of the fiber core). 18. A “parabolic-index fiber” is a specific case of a graded-index fiber in which the refractive index decreases quadratically with increasing distance from the center of the fiber core. 19. A “modulator” is a device that modulates an intensity or amplitude of an optical beam. 20. The term “optical power” is energy per unit time, as is delivered by a laser beam, for example.

Fiber for Varying Beam Characteristics

Disclosed herein are methods, systems, and apparatus configured to provide a fiber operable to provide a laser beam having variable beam characteristics (VBC) that may reduce cost, complexity, optical loss, or other drawbacks of the conventional methods described above. This VBC fiber is configured to vary a wide variety of optical beam characteristics. Such beam characteristics can be controlled using the VBC fiber thus allowing users to tune various beam characteristics to suit the particular requirements of an extensive variety of laser processing applications. For example, a VBC fiber may be used to tune: beam diameter, beam divergence distribution, BPP, intensity distribution, M² factor, NA, optical intensity, power density, radial beam position, radiance, spot size, or the like, or any combination thereof.

In general, the disclosed technology entails coupling a laser beam into a fiber in which the characteristics of the laser beam in the fiber can be adjusted by perturbing the laser beam and/or perturbing a first length of fiber by any of a variety of methods (e.g., bending the fiber or introducing one or more other perturbations) and fully or partially maintaining adjusted beam characteristics in a second length of fiber. The second length of fiber is specially configured to maintain and/or further modify the adjusted beam characteristics. In some cases, the second length of fiber preserves the adjusted beam characteristics through delivery of the laser beam to its ultimate use (e.g., materials processing). The first and second lengths of fiber may comprise the same or different fibers.

The disclosed technology is compatible with fiber lasers and fiber-coupled lasers. Fiber-coupled lasers typically deliver an output via a delivery fiber having a step-index refractive index profile (RIP), i.e., a flat or constant refractive index within the fiber core. In reality, the RIP of the delivery fiber may not be perfectly flat, depending on the design of the fiber. Important parameters are the fiber core diameter (d_(core)) and NA. The core diameter is typically in the range of 10-1000 micron (although other values are possible), and the NA is typically in the range of 0.06-0.22 (although other values are possible). A delivery fiber from the laser may be routed directly to the process head or work piece, or it may be routed to a fiber-to-fiber coupler (FFC) or fiber-to-fiber switch (FFS), which couples the light from the delivery fiber into a process fiber that transmits the beam to the process head or the work piece.

Most materials processing tools, especially those at high power (>1 kW), employ multimode (MM) fiber, but some employ single-mode (SM) fiber, which is at the lower end of the d_(core) and NA ranges. The beam characteristics from a SM fiber are uniquely determined by the fiber parameters. The beam characteristics from a MM fiber, however, can vary (unit-to-unit and/or as a function of laser power and time), depending on the beam characteristics from the laser source(s) coupled into the fiber, the launching or splicing conditions into the fiber, the fiber RIP, and the static and dynamic geometry of the fiber (bending, coiling, motion, micro-bending, etc.). For both SM and MM delivery fibers, the beam characteristics may not be optimum for a given materials processing task, and it is unlikely to be optimum for a range of tasks, motivating the desire to be able to systematically vary the beam characteristics in order to customize or optimize them for a particular processing task.

In one example, the VBC fiber may have a first length and a second length and may be configured to be interposed as an in-fiber device between the delivery fiber and the process head to provide the desired adjustability of the beam characteristics. To enable adjustment of the beam, a perturbation device and/or assembly is disposed in close proximity to and/or coupled with the VBC fiber and is responsible for perturbing the beam in a first length such that the beam's characteristics are altered in the first length of fiber, and the altered characteristics are preserved or further altered as the beam propagates in the second length of fiber. The perturbed beam is launched into a second length of the VBC fiber configured to conserve adjusted beam characteristics. The first and second lengths of fiber may be the same or different fibers and/or the second length of fiber may comprise a confinement fiber. The beam characteristics that are conserved by the second length of VBC fiber may include any of: beam diameter, beam divergence distribution, BPP, intensity distribution, luminance, M2 factor, NA, optical intensity, power density, radial beam position, radiance, spot size, or the like, or any combination thereof.

FIG. 1 illustrates an example VBC fiber 100 for providing a laser beam having variable beam characteristics without requiring the use of free-space optics to change the beam characteristics. VBC fiber 100 comprises a first length of fiber 104 and a second length of fiber 108. First length of fiber 104 and second length of fiber 108 may be the same or different fibers and may have the same or different RIPs. The first length of fiber 104 and the second length of fiber 108 may be joined together by a splice. First length of fiber 104 and second length of fiber 108 may be coupled in other ways, may be spaced apart, or may be connected via an interposing component such as another length of fiber, free-space optics, glue, index-matching material, or the like or any combination thereof.

A perturbation device 110 is disposed proximal to and/or envelops perturbation region 106. Perturbation device 110 may be a device, assembly, in-fiber structure, and/or other feature. Perturbation device 110 at least perturbs optical beam 102 in first length of fiber 104 or second length of fiber 108 or a combination thereof in order to adjust one or more beam characteristics of optical beam 102. Adjustment of beam 102 responsive to perturbation by perturbation device 110 may occur in first length of fiber 104 or second length of fiber 108 or a combination thereof. Perturbation region 106 may extend over various widths and may or may not extend into a portion of second length of fiber 108. As beam 102 propagates in VBC fiber 100, perturbation device 110 may physically act on VBC fiber 100 to perturb the fiber and adjust the characteristics of beam 102. Alternatively, perturbation device 110 may act directly on beam 102 to alter its beam characteristics. Subsequent to being adjusted, perturbed beam 112 has different beam characteristics than beam 102, which will be fully or partially conserved in second length of fiber 108. In another example, perturbation device 110 need not be disposed near a splice. Moreover, a splice may not be needed at all, for example VBC fiber 100 may be a single fiber, first length of fiber and second length of fiber could be spaced apart, or secured with a small gap (air-spaced or filled with an optical material, such as optical cement or an index-matching material).

Perturbed beam 112 is launched into second length of fiber 108, where perturbed beam 112 characteristics are largely maintained or continue to evolve as perturbed beam 112 propagates yielding the adjusted beam characteristics at the output of second length of fiber 108. In one example, the new beam characteristics may include an adjusted intensity distribution. In an example, an altered beam intensity distribution will be conserved in various structurally bounded confinement regions of second length of fiber 108. Thus, the beam intensity distribution may be tuned to a desired beam intensity distribution optimized for a particular laser processing task. In general, the intensity distribution of perturbed beam 112 will evolve as it propagates in the second length of fiber 108 to fill the confinement region(s) into which perturbed beam 112 is launched responsive to conditions in first length of fiber 104 and perturbation caused by perturbation device 110. In addition, the angular distribution may evolve as the beam propagates in the second fiber, depending on launch conditions and fiber characteristics. In general, fibers largely preserve the input divergence distribution, but the distribution can be broadened if the input divergence distribution is narrow and/or if the fiber has irregularities or deliberate features that perturb the divergence distribution. The various confinement regions, perturbations, and fiber features of second length of fiber 108 are described in greater detail below. Beams 102 and 112 are conceptual abstractions intended to illustrate how a beam may propagate through a VBC fiber 100 for providing variable beam characteristics and are not intended to closely model the behavior of a particular optical beam.

VBC fiber 100 may be manufactured by a variety of methods including PCVD (Plasma Chemical Vapor Deposition), OVD (Outside Vapor Deposition), VAD (Vapor Axial Deposition), MOCVD (Metal-Organic Chemical Vapor Deposition.) and/or DND (Direct Nanoparticle Deposition). VBC fiber 100 may comprise a variety of materials. For example, VBC fiber 100 may comprise Si0₂, Si0₂ doped with Ge0₂, germanosilicate, phosphorus pentoxide, phosphosilicate, Al₂0₃, aluminosilicate, or the like or any combinations thereof. Confinement regions may be bounded by cladding doped with fluorine, boron, or the like or any combinations thereof. Other dopants may be added to active fibers, including rare-earth ions such as Er³⁺ (erbium), Yb³⁺ (ytterbium), Nd³⁺ (neodymium), Tm³⁺ (thulium), Ho³⁺ (holmium), or the like or any combination thereof. Confinement regions may be bounded by cladding having a lower index than the confinement region with fluorine or boron doping. Alternatively, VBC fiber 100 may comprise photonic crystal fibers or micro-structured fibers.

VBC fiber 100 is suitable for use in any of a variety of fiber, fiber optic, or fiber laser devices, including continuous wave and pulsed fiber lasers, disk lasers, solid state lasers, or diode lasers (pulse rate unlimited except by physical constraints). Furthermore, implementations in a planar waveguide or other types of waveguides and not just fibers are within the scope of the claimed technology.

FIG. 2 depicts a cross-sectional view of an example VBC fiber 200 for adjusting beam characteristics of an optical beam. In an example, VBC fiber 200 may be a process fiber because it may deliver the beam to a process head for material processing. VBC fiber 200 comprises a first length of fiber 204 spliced at junction 206 to a second length of fiber 208. A perturbation assembly 210 is disposed proximal to junction 206. Perturbation assembly 210 may be any of a variety of devices configured to enable adjustment of the beam characteristics of an optical beam 202 propagating in VBC fiber 200. In an example, perturbation assembly 210 may be a mandrel and/or another device that may provide means of varying the bend radius and/or bend length of VBC fiber 200 near the splice. Other examples of perturbation devices are discussed below with respect to FIG. 24.

In an example, first length of fiber 204 has a parabolic-index RIP 212 as indicated by the left RIP graph. Most of the intensity distribution of beam 202 is concentrated in the center of fiber 204 when fiber 204 is straight or nearly straight. Second length of fiber 208 is a confinement fiber having RIP 214 as shown in the right RIP graph. Second length of fiber 208 includes confinement regions 216, 218 and 220. Confinement region 216 is a central core surrounded by two annular (or ring-shaped) confinement regions 218 and 220. Layers 222 and 224 are structural barriers of lower index material between confinement regions (216, 218 and 220), commonly referred to as “cladding” regions. In one example, layers 222 and 224 may comprise rings of fluorosilicate; in some embodiments, the fluorosilicate cladding layers are relatively thin. Other materials may be used as well and claimed subject matter is not limited in this regard.

In an example, as beam 202 propagates along VBC fiber 200, perturbation assembly 210 may physically act on fiber 208 and/or beam 202 to adjust its beam characteristics and generate adjusted beam 226. In the current example, the intensity distribution of beam 202 is modified by perturbation assembly 210. Subsequent to adjustment of beam 202 the intensity distribution of adjusted beam 226 may be concentrated in outer confinement regions 218 and 220 with relatively little intensity in the central confinement region 216. Because each of confinement regions 216, 218, and/or 220 is isolated by the thin layers of lower index material in barrier layers 222 and 224, second length of fiber 208 can substantially maintain the adjusted intensity distribution of adjusted beam 226. The beam will typically become distributed azimuthally within a given confinement region but will not transition (significantly) between the confinement regions as it propagates along the second length of fiber 208. Thus, the adjusted beam characteristics of adjusted beam 226 are largely conserved within the isolated confinement regions 216, 218, and/or 220. In some cases, it be may desirable to have the beam 226 power divided among the confinement regions 216, 218, and/or 220 rather than concentrated in a single region, and this condition may be achieved by generating an appropriately adjusted beam 226.

In one example, core confinement region 216 and annular confinement regions 218 and 220 may be composed of fused silica glass, and cladding 222 and 224 defining the confinement regions may be composed of fluorosilicate glass. Other materials may be used to form the various confinement regions (216, 218 and 220), including germanosilicate, phosphosilicate, aluminosilicate, or the like, or a combination thereof and claimed subject matter is not so limited. Other materials may be used to form the barrier rings (222 and 224), including fused silica, borosilicate, or the like or a combination thereof, and claimed subject matter is not so limited. In other embodiments, the optical fibers or waveguides include or are composed of various polymers or plastics or crystalline materials. Generally, the core confinement regions have refractive indices that are greater than the refractive indices of adjacent barrier/cladding regions.

In some examples, it may be desirable to increase a number of confinement regions in a second length of fiber to increase granularity of beam control over beam displacements for fine-tuning a beam profile. For example, confinement regions may be configured to provide stepwise beam displacement.

FIG. 3 illustrates an example method of perturbing fiber 200 for providing variable beam characteristics of an optical beam. Changing the bend radius of a fiber may change the radial beam position, divergence angle, and/or radiance profile of a beam within the fiber. The bend radius of VBC fiber 200 can be decreased from a first bend radius R₁ to a second bend radius R₂ about splice junction 206 by using a stepped mandrel or cone as the perturbation assembly 210. Additionally or alternatively, the engagement length on the mandrel(s) or cone can be varied. Rollers 250 may be employed to engage VBC fiber 200 across perturbation assembly 210. In an example, an amount of engagement of rollers 250 with fiber 200 has been shown to shift the distribution of the intensity profile to the outer confinement regions 218 and 220 of fiber 200 with a fixed mandrel radius. There are a variety of other methods for varying the bend radius of fiber 200, such as using a clamping assembly, flexible tubing, or the like, or a combination thereof, and claimed subject matter is not limited in this regard. In another example, for a particular bend radius the length over which VBC fiber 200 is bent can also vary beam characteristics in a controlled and reproducible way. In examples, changing the bend radius and/or length over which the fiber is bent at a particular bend radius also modifies the intensity distribution of the beam such that one or more modes may be shifted radially away from the center of a fiber core.

Maintaining the bend radius of the fibers across junction 206 ensures that the adjusted beam characteristics such as radial beam position and radiance profile of optical beam 202 will not return to beam 202′s unperturbed state before being launched into second length of fiber 208. Moreover, the adjusted radial beam characteristics, including position, divergence angle, and/or intensity distribution, of adjusted beam 226 can be varied based on an extent of decrease in the bend radius and/or the extent of the bent length of VBC fiber 200. Thus, specific beam characteristics may be obtained using this method.

In the current example, first length of fiber 204 having first RIP 212 is spliced at junction 206 to a second length of fiber 208 having a second RIP 214. However, it is possible to use a single fiber having a single RIP formed to enable perturbation (e.g., by micro-bending) of the beam characteristics of beam 202 and also to enable conservation of the adjusted beam. Such a RIP may be similar to the RIPs shown in fibers illustrated in FIGS. 17, 18, and/or 19.

FIGS. 7-10 provide experimental results for VBC fiber 200 (shown in FIGS. 2 and 3) and illustrate further a beam response to perturbation of VBC fiber 200 when a perturbation assembly 210 acts on VBC fiber 200 to bend the fiber. FIGS. 4-6 are simulations and FIGS. 7-10 are experimental results wherein a beam from a SM 1050 nm source was launched into an input fiber (not shown) with a 40 micron core diameter. The input fiber was spliced to first length of fiber 204.

FIG. 4 is an example graph 400 illustrating the calculated profile of the lowest-order mode (LP₀₁) for a first length of fiber 204 for different fiber bend radii 402, wherein a perturbation assembly 210 involves bending VBC fiber 200. As the fiber bend radius is decreased, an optical beam propagating in VBC fiber 200 is adjusted such that the mode shifts radially away from the center 404 of a VBC fiber 200 core (r=0 micron) toward the core/cladding interface (located at r=100 micron in this example). Higher-order modes (LP_(1n)) also shift with bending. Thus, a straight or nearly straight fiber (very large bend radius), curve 406 for LP₀₁ is centered at or near the center of VBC fiber 200. At a bend radius of about 6 cm, curve 408 for LP₀₁ is shifted to a radial position of about 40 μm from the center 406 of VBC fiber 200. At a bend radius of about 5 cm, curve 410 for LP₀₁ is shifted to a radial position about 50 μm from the center 406 of VBC fiber 200. At a bend radius of about 4 cm, curve 412 for LP₀₁ is shifted to a radial position about 60 μm from the center 406 of VBC fiber 200. At a bend radius of about 3 cm, curve 414 for LP₀₁ is shifted to a radial position about 80 μm from the center 406 of VBC fiber 200. At a bend radius of about 2.5 cm, a curve 416 for LP₀₁ is shifted to a radial position about 85 μm from the center 406 of VBC fiber 200. Note that the shape of the mode remains relatively constant (until it approaches the edge of the core), which is a specific property of a parabolic RIP. Although, this property may be desirable in some situations, it is not required for the VBC functionality, and other RIPs may be employed.

In an example, if VBC fiber 200 is straightened, LP₀₁ mode will shift back toward the center of the fiber. Thus, the purpose of second length of fiber 208 is to “trap” or confine the adjusted intensity distribution of the beam in a confinement region that is displaced from the center of the VBC fiber 200. The splice between fibers 204 and 208 is included in the bent region, thus the shifted mode profile will be preferentially launched into one of the ring-shaped confinement regions 218 and 220 or be distributed among the confinement regions. FIGS. 5 and 6 illustrate this effect.

FIG. 5 illustrates an example two-dimensional intensity distribution at junction 206 within second length of fiber 208 when VBC fiber 200 is nearly straight. A significant portion of LP₀₁ and LP_(1n) are within confinement region 216 of fiber 208. FIG. 6 illustrates the two-dimensional intensity distribution at junction 206 within second length of fiber 208 when VBC fiber 200 is bent with a radius chosen to preferentially excite confinement region 220 (the outermost confinement region) of second length of fiber 208. A significant portion of LP₀₁ and LP_(1n) are within confinement region 220 of fiber 208.

In an example, second length of fiber 208 confinement region 216 has a 100 micron diameter, confinement region 218 is between 120 micron and 200 micron in diameter, and confinement region 220 is between 220 micron and 300 micron diameter. Confinement regions 216, 218, and 220 are separated by 10 um thick rings of fluorosilicate, providing an NA of 0.22 for the confinement regions. Other inner and outer diameters for the confinement regions, thicknesses of the rings separating the confinement regions, NA values for the confinement regions, and numbers of confinement regions may be employed.

Referring again to FIG. 5, with the noted parameters, when VBC fiber 200 is straight about 90% of the power is contained within the central confinement region 216, and about 100% of the power is contained within confinement regions 216 and 218. Referring now to FIG. 6, when fiber 200 is bent to preferentially excite second ring confinement region 220, nearly 75% of the power is contained within confinement region 220, and more than 95% of the power is contained within confinement regions 218 and 220. These calculations include LP₀₁ and two higher-order modes, which is typical in some 2-4 kW fiber lasers.

It is clear from FIGS. 5 and 6 that in the case where a perturbation assembly 210 acts on VBC fiber 200 to bend the fiber, the bend radius determines the spatial overlap of the modal intensity distribution of the first length of fiber 204 with the different guiding confinement regions (216, 218, and 220) of the second length of fiber 208. Changing the bend radius can thus change the intensity distribution at the output of the second length of fiber 208, thereby changing the diameter or spot size of the beam, and thus also changing its radiance and BPP value. This adjustment of the spot size may be accomplished in an all-fiber structure, involving no free-space optics and consequently may reduce or eliminate the disadvantages of free-space optics discussed above. Such adjustments can also be made with other perturbation assemblies that alter bend radius, bend length, fiber tension, temperature, or other perturbations discussed below.

In a typical materials processing system (e.g., a cutting or welding tool), the output of the process fiber is imaged at or near the work piece by the process head. Varying the intensity distribution as shown in FIGS. 5 and 6 thus enables variation of the beam profile at the work piece in order to tune and/or optimize the process, as desired. Specific RIPs for the two fibers were assumed for the purpose of the above calculations, but other RIPs are possible, and claimed subject matter is not limited in this regard.

FIGS. 7-10 depict experimental results (measured intensity distributions) to illustrate further output beams for various bend radii of VBC fiber 200 shown in FIG. 2.

In FIG. 7 when VBC fiber 200 is straight, the beam is nearly completely confined to confinement region 216. As the bend radius is decreased, the intensity distribution shifts to higher diameters (FIGS. 8-10). FIG. 8 depicts the intensity distribution when the bend radius of VBC fiber 200 is chosen to shift the intensity distribution preferentially to confinement region 218. FIG. 9 depicts the experimental results when the bend radius is further reduced and chosen to shift the intensity distribution outward to confinement region 220 and confinement region 218. In FIG. 10, at the smallest bend radius, the beam is nearly a “donut mode”, with most of the intensity in the outermost confinement region 220.

Despite excitation of the confinement regions from one side at the splice junction 206, the intensity distributions are nearly symmetric azimuthally because of scrambling within confinement regions as the beam propagates within the VBC fiber 200. Although the beam will typically scramble azimuthally as it propagates, various structures or perturbations (e.g., coils) could be included to facilitate this process.

For the fiber parameters used in the experiment shown in FIGS. 7-10, particular confinement regions were not exclusively excited because some intensity was present in multiple confinement regions. This feature may enable advantageous materials processing applications that are optimized by having a flatter or distributed beam intensity distribution. In applications requiring cleaner excitation of a given confinement region, different fiber RIPs could be employed to enable this feature.

The results shown in FIGS. 7-10 pertain to the particular fibers used in this experiment, and the details will vary depending on the specifics of the implementation. In particular, the spatial profile and divergence distribution of the output beam and their dependence on bend radius will depend on the specific RIPs employed, on the splice parameters, and on the characteristics of the laser source launched into the first fiber.

Different fiber parameters than those shown in FIG. 2 may be used and still be within the scope of the claimed subject matter. Specifically, different RIPs and core sizes and shapes may be used to facilitate compatibility with different input beam profiles and to enable different output beam characteristics. Example RIPs for the first length of fiber, in addition to the parabolic-index profile shown in FIG. 2, include other graded-index profiles, step-index, pedestal designs (i.e., nested cores with progressively lower refractive indices with increasing distance from the center of the fiber), and designs with nested cores with the same refractive index value but with various NA values for the central core and the surrounding rings. Example RIPs for the second length of fiber, in addition to the profile shown in FIG. 2, include confinement fibers with different numbers of confinement regions, non-uniform confinement-region thicknesses, different and/or non-uniform values for the thicknesses of the rings surrounding the confinement regions, different and/or non-uniform NA values for the confinement regions, different refractive-index values for the high-index and low-index portions of the RIP, non-circular confinement regions (such as elliptical, oval, polygonal, square, rectangular, or combinations thereof), as well as other designs as discussed in further detail with respect to FIGS. 26-28. Furthermore, VBC fiber 200 and other examples of a VBC fiber described herein are not restricted to use of two fibers. In some examples, implementation may include use of one fiber or more than two fibers. In some cases, the fiber(s) may not be axially uniform; for example, they could include fiber Bragg gratings or long-period gratings, or the diameter could vary along the length of the fiber. In addition, the fibers do not have to be azimuthally symmetric, e.g., the core(s) could have square or polygonal shapes. Various fiber coatings (buffers) may be employed, including high-index or index-matched coatings (which strip light at the glass-polymer interface) and low-index coatings (which guide light by total internal reflection at the glass-polymer interface). In some examples, multiple fiber coatings may be used on VBC fiber 200.

FIGS. 11-16 illustrate cross-sectional views of examples of first lengths of fiber for enabling adjustment of beam characteristics in a VBC fiber responsive to perturbation of an optical beam propagating in the first lengths of fiber. Some examples of beam characteristics that may be adjusted in the first length of fiber are: beam diameter, beam divergence distribution, BPP, intensity distribution, luminance, M² factor, NA, optical intensity profile, power density profile, radial beam position, radiance, spot size, or the like, or any combination thereof. The first lengths of fiber depicted in FIGS. 11-16 and described below are merely examples and do not provide an exhaustive recitation of the variety of first lengths of fiber that may be utilized to enable adjustment of beam characteristics in a VBC fiber assembly. Selection of materials, appropriate RIPs, and other variables for the first lengths of fiber illustrated in FIGS. 11-16 at least depend on a desired beam output. A wide variety of fiber variables are contemplated and are within the scope of the claimed subject matter. Thus, claimed subject matter is not limited by examples provided herein.

In FIG. 11 first length of fiber 1100 comprises a step-index profile 1102. FIG. 12 illustrates a first length of fiber 1200 comprising a “pedestal RIP” (i.e., a core comprising a step-index region surrounded by a larger step-index region) 1202. FIG. 13 illustrates first length of fiber 1300 comprising a multiple-pedestal RIP 1302.

FIG. 14A illustrates first length of fiber 1400 comprising a graded-index profile 1418 surrounded by a down-doped region 1404. When the fiber 1400 is perturbed, modes may shift radially outward in fiber 1400 (e.g., during bending of fiber 1400). Graded-index profile 1402 may be designed to promote maintenance or even compression of modal shape. This design may promote adjustment of a beam propagating in fiber 1400 to generate a beam having a beam intensity distribution concentrated in an outer perimeter of the fiber (i.e., in a portion of the fiber core that is displaced from the fiber axis). As described above, when the adjusted beam is coupled into a second length of fiber having confinement regions, the intensity distribution of the adjusted beam may be trapped in the outermost confinement region, providing a donut shaped intensity distribution. A beam spot having a narrow outer confinement region may be useful to enable certain material processing actions.

FIG. 14B illustrates first length of fiber 1406 comprising a graded-index profile 1414 surrounded by a down-doped region 1408 similar to fiber 1400. However, fiber 1406 includes a divergence structure 1410 (a lower-index region) as can be seen in profile 1412. The divergence structure 1410 is an area of material with a lower refractive index than that of the surrounding core. As the beam is launched into first length of fiber 1406, refraction from divergence structure 1410 causes the beam divergence to increase in first length of fiber 1406. The amount of increased divergence depends on the amount of spatial overlap of the beam with the divergence structure 1410 and the magnitude of the index difference between the divergence structure 1410 and the core material. Divergence structure 1410 can have a variety of shapes, depending on the input divergence distribution and desired output divergence distribution. In an example, divergence structure 1410 has a triangular or graded index shape.

FIG. 15 illustrates a first length of fiber 1500 comprising a parabolic-index central region 1502 surrounded by a constant-index region 1504, and the constant-index region 1504 is surrounded by a lower-index annular layer 1506. The lower-index annulus 1506 helps guide a beam propagating in fiber 1500. When the propagating beam is perturbed, modes shift radially outward in fiber 1500 (e.g., during bending of fiber 1500). As one or more modes shift radially outward, parabolic-index region 1502 promotes retention of modal shape. When the modes reach the constant-index region of the RIP 1510, they will be compressed against the low-index ring 1506, which may cause preferential excitation of the outermost confinement region in the second fiber (in comparison to the first fiber RIP shown in FIG. 14). In one implementation, this fiber design works with a confinement fiber having a central step-index core and a single annular core. The parabolic-index portion 1502 of the RIP overlaps with the central step-index core of the confinement fiber. The constant-index portion 1504 overlaps with the annular core of the confinement fiber. The constant-index portion 1504 of the first fiber is intended to make it easier to move the beam into overlap with the annular core by bending. This fiber design also works with other designs of the confinement fiber.

FIG. 16 illustrates a first length of fiber 1600 comprising guiding regions 1604, 1606, 1608, and 1616 bounded by lower-index layers 1610, 1612, and 1614 where the indexes of the lower-index layers 1610, 1612, and 1614 are stepped or, more generally, do not all have the same value. The stepped-index layers may serve to bound the beam intensity to certain guiding regions (1604, 1606, 1608, and 1616) when the perturbation assembly 210 (see FIG. 2) acts on the fiber 1600. In this way, adjusted beam light may be trapped in the guiding regions over a range of perturbation actions (such as over a range of bend radii, a range of bend lengths, a range of micro-bending pressures, and/or a range of acousto-optical signals), allowing for a certain degree of perturbation tolerance before a beam intensity distribution is shifted to a more distant radial position in fiber 1600. Thus, variation in beam characteristics may be controlled in a step-wise fashion. The radial widths of the guiding regions 1604, 1606, 1608, and 1616 may be adjusted to achieve a desired ring width, as may be required by an application. Also, a guiding region can have a thicker radial width to facilitate trapping of a larger fraction of the incoming beam profile if desired. Region 1606 is an example of such a design.

FIGS. 17-21 depict examples of fibers configured to enable maintenance and/or confinement of adjusted beam characteristics in the second length of fiber (e.g., fiber 208). These fiber designs are referred to as “ring-shaped confinement fibers” because they contain a central core surrounded by annular or ring-shaped cores. These designs are merely examples and not an exhaustive recitation of the variety of fiber RIPs that may be used to enable maintenance and/or confinement of adjusted beam characteristics within a fiber. Thus, claimed subject matter is not limited to the examples provided herein. Moreover, any of the first lengths of fiber described above with respect to FIGS. 11-16 may be combined with any of the second length of fiber described FIGS. 17-21.

FIG. 17 illustrates a cross-sectional view of an example second length of fiber for maintaining and/or confining adjusted beam characteristics in a VBC fiber assembly. As the perturbed beam is coupled from a first length of fiber to second length of fiber 1700, the second length of fiber 1700 may maintain at least a portion of the beam characteristics adjusted in response to perturbation in the first length of fiber within one or more of confinement regions 1704, 1706, and/or 1708. Fiber 1700 has a RIP 1702. Each of confinement regions 1704, 1706, and/or 1708 is bounded by a lower index layer 1710 and/or 1712. This design enables second length of fiber 1700 to maintain the adjusted beam characteristics. As a result, a beam output by fiber 1700 will substantially maintain the received adjusted beam as modified in the first length of fiber giving the output beam adjusted beam characteristics, which may be customized to a processing task or other application.

Similarly, FIG. 18 depicts a cross-sectional view of an example second length of fiber 1800 for maintaining and/or confining beam characteristics adjusted in response to perturbation in the first length of fiber in a VBC fiber assembly. Fiber 1800 has a RIP 1802. However, confinement regions 1808, 1810, and/or 1812 have different thicknesses than confinement regions 1704, 1706, and 1708. Each of confinement regions 1808, 1810, and/or 1812 is bounded by a lower index layer 1804 and/or 1806. Varying the thicknesses of the confinement regions (and/or barrier regions) enables tailoring or optimization of a confined adjusted radiance profile by selecting particular radial positions within which to confine an adjusted beam.

FIG. 19 depicts a cross-sectional view of an example second length of fiber 1900 having a RIP 1902 for maintaining and/or confining an adjusted beam in a VBC fiber assembly configured to provide variable beam characteristics. In this example, the number and thicknesses of confinement regions 1904, 1906, 1908, and 1910 are different from fiber 1700 and 1800 and the barrier layers 1912, 1914, and 1916 are of varied thicknesses as well. Furthermore, confinement regions 1904, 1906, 1908, and 1910 have different indexes of refraction and barrier layers 1912, 1914, and 1916 have different indexes of refraction as well. This design may further enable a more granular or optimized tailoring of the confinement and/or maintenance of an adjusted beam radiance to particular radial locations within fiber 1900. As the perturbed beam is launched from a first length of fiber to second length of fiber 1900 the modified beam characteristics of the beam (having an adjusted intensity distribution, radial position, and/or divergence angle, or the like, or a combination thereof) is confined within a specific radius by one or more of confinement regions 1904, 1906, 1908 and/or 1910 of second length of fiber 1900.

As noted previously, the divergence angle of a beam may be conserved or adjusted and then conserved in the second length of fiber. There are a variety of methods to change the divergence angle of a beam. The following are examples of fibers configured to enable adjustment of the divergence angle of a beam propagating from a first length of fiber to a second length of fiber in a fiber assembly for varying beam characteristics. However, these are merely examples and not an exhaustive recitation of the variety of methods that may be used to enable adjustment of divergence of a beam. Thus, claimed subject matter is not limited to the examples provided herein.

FIG. 20 depicts a cross-sectional view of an example second length of fiber 2000 having RIP 2002 for modifying, maintaining, and/or confining beam characteristics adjusted in response to perturbation in the first length of fiber. In this example, second length of fiber 2000 is similar to the previously described second lengths of fiber and forms a portion of the VBC fiber assembly for delivering variable beam characteristics as discussed above. There are three confinement regions 2004, 2006, and 2008 and three barrier layers 2010, 2012, and 2016. Second length of fiber 2000 also has a divergence structure 2014 situated within the confinement region 2006. The divergence structure 2014 is an area of material with a lower refractive index than that of the surrounding confinement region. As the beam is launched into second length of fiber 2000 refraction from divergence structure 2014 causes the beam divergence to increase in second length of fiber 2000. The amount of increased divergence depends on the amount of spatial overlap of the beam with the divergence structure 2014 and the magnitude of the index difference between the divergence structure 2014 and the core material. By adjusting the radial position of the beam near the launch point into the second length of fiber 2000, the divergence distribution may be varied. The adjusted divergence of the beam is conserved in fiber 2000, which is configured to deliver the adjusted beam to the process head, another optical system (e.g., fiber-to-fiber coupler or fiber-to-fiber switch), the work piece, or the like, or a combination thereof. In an example, divergence structure 2014 may have an index dip of about 10⁻⁵-3×10⁻² with respect to the surrounding material. Other values of the index dip may be employed within the scope of this disclosure and claimed subject matter is not so limited.

FIG. 21 depicts a cross-sectional view of an example second length of fiber 2100 having a RIP 2102 for modifying, maintaining, and/or confining beam characteristics adjusted in response to perturbation in the first length of fiber. Second length of fiber 2100 forms a portion of a VBC fiber assembly for delivering a beam having variable characteristics. In this example, there are three confinement regions 2104, 2106, and 2108 and three barrier layers 2110, 2112, and 2116. Second length of fiber 2100 also has a plurality of divergence structures 2114 and 2118. The divergence structures 2114 and 2118 are areas of graded lower index material. As the beam is launched from the first length fiber into second length of fiber 2100, refraction from divergence structures 2114 and 2118 causes the beam divergence to increase. The amount of increased divergence depends on the amount of spatial overlap of the beam with the divergence structure and the magnitude of the index difference between the divergence structure 2114 and/or 2118 and the surrounding core material of confinement regions 2106 and 2104 respectively. By adjusting the radial position of the beam near the launch point into the second length of fiber 2100, the divergence distribution may be varied. The design shown in FIG. 21 allows the intensity distribution and the divergence distribution to be varied somewhat independently by selecting both a particular confinement region and the divergence distribution within that conferment region (because each confinement region may include a divergence structure). The adjusted divergence of the beam is conserved in fiber 2100, which is configured to deliver the adjusted beam to the process head, another optical system, or the work piece. Forming the divergence structures 2114 and 2118 with a graded or non-constant index enables tuning of the divergence profile of the beam propagating in fiber 2100. An adjusted beam characteristic such as a radiance profile and/or divergence profile may be conserved as it is delivered to a process head by the second fiber. Alternatively, an adjusted beam characteristic such as a radiance profile and/or divergence profile may be conserved or further adjusted as it is routed by the second fiber through a fiber-to-fiber coupler (FFC) and/or fiber-to-fiber switch (FFS) and to a process fiber, which delivers the beam to the process head or the work piece.

FIGS. 26-28 are cross-sectional views illustrating examples of fibers and fiber RIPs configured to enable maintenance and/or confinement of adjusted beam characteristics of a beam propagating in an azimuthally asymmetric second length of fiber wherein the beam characteristics are adjusted responsive to perturbation of a first length of fiber coupled to the second length of fiber and/or perturbation of the beam by a perturbation device 110. These azimuthally asymmetric designs are merely examples and are not an exhaustive recitation of the variety of fiber RIPs that may be used to enable maintenance and/or confinement of adjusted beam characteristics within an azimuthally asymmetric fiber. Thus, claimed subject matter is not limited to the examples provided herein. Moreover, any of a variety of first lengths of fiber (e.g., like those described above) may be combined with any azimuthally asymmetric second length of fiber (e.g., like those described in FIGS. 26-28).

FIG. 26 illustrates RIPs at various azimuthal angles of a cross-section through an elliptical fiber 2600. At a first azimuthal angle 2602, fiber 2600 has a first RIP 2604. At a second azimuthal angle 2606 that is rotated 45° from first azimuthal angle 2602, fiber 2600 has a second RIP 2608. At a third azimuthal angle 2610 that is rotated another 45° from second azimuthal angle 2606, fiber 2600 has a third RIP 2612. First, second and third RIPs 2604, 2608 and 2612 are all different.

FIG. 27 illustrates RIPs at various azimuthal angles of a cross-section through a multicore fiber 2700. At a first azimuthal angle 2702, fiber 2700 has a first RIP 2704. At a second azimuthal angle 2706, fiber 2700 has a second RIP 2708. First and second RIPs 2704 and 2708 are different. In an example, perturbation device 110 may act in multiple planes in order to launch the adjusted beam into different regions of an azimuthally asymmetric second fiber.

FIG. 28 illustrates RIPs at various azimuthal angles of a cross-section through a fiber 2800 having at least one crescent shaped core. In some cases, the corners of the crescent may be rounded, flattened, or otherwise shaped, which may minimize optical loss. At a first azimuthal angle 2802, fiber 2800 has a first RIP 2804. At a second azimuthal angle 2806, fiber 2800 has a second RIP 2808. First and second RIPs 2804 and 2808 are different.

FIG. 22A illustrates an example of a laser system 2200 including a VBC fiber assembly 2202 configured to provide variable beam characteristics. VBC fiber assembly 2202 comprises a first length of fiber 104, second length of fiber 108, and a perturbation device 110. VBC fiber assembly 2202 is disposed between feeding fiber 2212 (i.e., the output fiber from the laser source) and VBC delivery fiber 2240. VBC delivery fiber 2240 may comprise second length of fiber 108 or an extension of second length of fiber 108 that modifies, maintains, and/or confines adjusted beam characteristics. Beam 2210 is coupled into VBC fiber assembly 2202 via feeding fiber 2212. Fiber assembly 2202 is configured to vary the characteristics of beam 2210 in accordance with the various examples described above. The output of fiber assembly 2202 is adjusted beam 2214 which is coupled into VBC delivery fiber 2240. VBC delivery fiber 2240 delivers adjusted beam 2214 to free-space optics assembly 2208, which then couples beam 2214 into a process fiber 2204. Adjusted beam 2214 is then delivered to process head 2206 by process fiber 2204. The process head can include guided wave optics (such as fibers and fiber coupler), free space optics such as lenses, mirrors, optical filters, diffraction gratings), beam scan assemblies such as galvanometer scanners, polygonal mirror scanners, or other scanning systems that are used to shape the beam 2214 and deliver the shaped beam to a workpiece.

In laser system 2200, one or more of the free-space optics of assembly 2208 may be disposed in an FFC or other beam coupler 2216 to perform a variety of optical manipulations of an adjusted beam 2214 (represented in FIG. 22A with different dashing than beam 2210). For example, free-space optics assembly 2208 may preserve the adjusted beam characteristics of beam 2214. Process fiber 2204 may have the same RIP as VBC delivery fiber 2240. Thus, the adjusted beam characteristics of adjusted beam 2214 may be preserved all the way to process head 2206. Process fiber 2204 may comprise a RIP similar to any of the second lengths of fiber described above, including confinement regions.

Alternatively, as illustrated in FIG. 22B, free-space optics assembly 2208 may change the adjusted beam characteristics of beam 2214 by, for example, increasing or decreasing the divergence and/or the spot size of beam 2214 (e.g., by magnifying or demagnifying beam 2214) and/or otherwise further modifying adjusted beam 2214. Furthermore, process fiber 2204 may have a different RIP than VBC delivery fiber 2240. Accordingly, the RIP of process fiber 2204 may be selected to preserve additional adjustment of adjusted beam 2214 made by the free-space optics of assembly 2208 to generate a twice adjusted beam 2224 (represented in FIG. 228 with different dashing than beam 2214).

FIG. 23 illustrates an example of a laser system 2300 including VBC fiber assembly 2302 disposed between feeding fiber 2312 and VBC delivery fiber 2340. During operation, beam 2310 is coupled into VBC fiber assembly 2302 via feeding fiber 2312. Fiber assembly 2302 includes a first length of fiber 104, second length of fiber 108, and a perturbation device 110 and is configured to vary characteristics of beam 2310 in accordance with the various examples described above. Fiber assembly 2302 generates adjusted beam 2314 output by VBC delivery fiber 2340. VBC delivery fiber 2340 comprises a second length of fiber 108 of fiber for modifying, maintaining, and/or confining adjusted beam characteristics in a fiber assembly 2302 in accordance with the various examples described above (see FIGS. 17-21, for example). VBC delivery fiber 2340 couples adjusted beam 2314 into beam switch (FFS) 2332, which then couples its various output beams to one or more of multiple process fibers 2304, 2320, and 2322. Process fibers 2304, 2320, and 2322 deliver adjusted beams 2314, 2328, and 2330 to respective process heads 2306, 2324, and 2326.

In an example, beam switch 2332 includes one or more sets of free-space optics 2308, 2316, and 2318 configured to perform a variety of optical manipulations of adjusted beam 2314. Free-space optics 2308, 2316, and 2318 may preserve or vary adjusted beam characteristics of beam 2314. Thus, adjusted beam 2314 may be maintained by the free-space optics or adjusted further. Process fibers 2304, 2320, and 2322 may have the same or a different RIP as VBC delivery fiber 2340, depending on whether it is desirable to preserve or further modify a beam passing from the free-space optics assemblies 2308, 2316, and 2318 to respective process fibers 2304, 2320, and 2322. In other examples, one or more beam portions of beam 2310 are coupled to a workpiece without adjustment, or different beam portions are coupled to respective VBC fiber assemblies so that beam portions associated with a plurality of beam characteristics can be provided for simultaneous workpiece processing. Alternatively, beam 2310 can be switched to one or more of a set of VBC fiber assemblies.

Routing adjusted beam 2314 through any of free-space optics assemblies 2308, 2316, and 2318 enables delivery of a variety of additionally adjusted beams to process heads 2206, 2324, and 2326. Therefore, laser system 2300 provides additional degrees of freedom for varying the characteristics of a beam, as well as switching the beam between process heads (“time sharing”) and/or delivering the beam to multiple process heads simultaneously (“power sharing”).

For example, free-space optics in beam switch 2332 may direct adjusted beam 2314 to free-space optics assembly 2316 configured to preserve the adjusted characteristics of beam 2314. Process fiber 2304 may have the same RIP as VBC delivery fiber 2340. Thus, the beam delivered to process head 2306 will be a preserved adjusted beam 2314.

In another example, beam switch 2332 may direct adjusted beam 2314 to free-space optics assembly 2318 configured to preserve the adjusted characteristics of adjusted beam 2314. Process fiber 2320 may have a different RIP than VBC delivery fiber 2340 and may be configured with divergence altering structures as described with respect to FIGS. 20 and 21 to provide additional adjustments to the divergence distribution of beam 2314. Thus, the beam delivered to process head 2324 will be a twice adjusted beam 2328 having a different beam divergence profile than adjusted beam 2314.

Process fibers 2304, 2320, and/or 2322 may comprise a RIP similar to any of the second lengths of fiber described above, including confinement regions or a wide variety of other RIPs, and claimed subject matter is not limited in this regard.

In yet another example, free-space optics switch 2332 may direct adjusted beam 2314 to free-space optics assembly 2308 configured to change the beam characteristics of adjusted beam 2314. Process fiber 2322 may have a different RIP than VBC delivery fiber 2340 and may be configured to preserve (or alternatively further modify) the new further adjusted characteristics of beam 2314. Thus, the beam delivered to process head 2326 will be a twice adjusted beam 2330 having different beam characteristics (due to the adjusted divergence profile and/or intensity profile) than adjusted beam 2314.

In FIGS. 22A, 22B, and 23, the optics in the FFC or FFS may adjust the spatial profile and/or divergence profile by magnifying or demagnifying the beam 2214 before launching into the process fiber. They may also adjust the spatial profile and/or divergence profile via other optical transformations. They may also adjust the launch position into the process fiber. These methods may be used alone or in combination.

FIGS. 22A, 22B, and 23 merely provide examples of combinations of adjustments to beam characteristics using free-space optics and various combinations of fiber RIPs to preserve or modify adjusted beams 2214 and 2314. The examples provided above are not exhaustive and are meant for illustrative purposes only. Thus, claimed subject matter is not limited in this regard.

FIG. 24 illustrates various examples of perturbation devices, assemblies or methods (for simplicity referred to collectively herein as “perturbation device 110”) for perturbing a VBC fiber 200 and/or an optical beam propagating in VBC fiber 200 according to various examples provided herein. Perturbation device 110 may be any of a variety of devices, methods, and/or assemblies configured to enable adjustment of beam characteristics of a beam propagating in VBC fiber 200. In an example, perturbation device 110 may be a mandrel 2402, a micro-bend 2404 in the VBC fiber, flexible tubing 2406, an acousto-optic transducer 2408, a thermal device 2410, a piezo-electric device 2412, a grating 2414, a clamp 2416 (or other fastener), or the like, or any combination thereof. These are merely examples of perturbation devices 110 and not an exhaustive listing of perturbation devices 110 and claimed subject matter is not limited in this regard.

Mandrel 2402 may be used to perturb VBC fiber 200 by providing a form about which VBC fiber 200 may be bent. As discussed above, reducing the bend radius of VBC fiber 200 moves the intensity distribution of the beam radially outward. In some examples, mandrel 2402 may be stepped or conically shaped to provide discrete bend radii levels. Alternatively, mandrel 2402 may comprise a cone shape without steps to provide continuous bend radii for more granular control of the bend radius. The radius of curvature of mandrel 2402 may be constant (e.g., a cylindrical form) or non-constant (e.g., an oval-shaped form). Similarly, flexible tubing 2406, clamps 2416 (or other varieties of fasteners), or rollers 250 may be used to guide and control the bending of VBC fiber 200 about mandrel 2402. Furthermore, changing the length over which the fiber is bent at a particular bend radius also may modify the intensity distribution of the beam. VBC fiber 200 and mandrel 2402 may be configured to change the intensity distribution within the first fiber predictably (e.g., in proportion to the length over which the fiber is bent and/or the bend radius). Rollers 250 may move up and down along a track 2442 on platform 2434 to change the bend radius of VBC fiber 200.

Clamps 2416 (or other fasteners) may be used to guide and control the bending of VBC fiber 200 with or without a mandrel 2402. Clamps 2416 may move up and down along a track 2442 or platform 2446. Clamps 2416 may also swivel to change bend radius, tension, or direction of VBC fiber 200. Controller 2448 may control the movement of clamps 2416.

In another example, perturbation device 110 may be flexible tubing 2406 and may guide bending of VBC fiber 200 with or without a mandrel 2402. Flexible tubing 2406 may encase VBC fiber 200. Tubing 2406 may be made of a variety of materials and may be manipulated using piezoelectric transducers controlled by controller 2444. In another example, clamps or other fasteners may be used to move flexible tubing 2406.

Micro-bend 2404 in VBC fiber is a local perturbation caused by lateral mechanical stress on the fiber. Micro-bending can cause mode coupling and/or transitions from one confinement region to another confinement region within a fiber, resulting in varied beam characteristics of the beam propagating in a VBC fiber 200. Mechanical stress may be applied by an actuator 2436 that is controlled by controller 2440. However, this is merely an example of a method for inducing mechanical stress in fiber 200 and claimed subject matter is not limited in this regard.

Acousto-optic transducer (AOT) 2408 may be used to induce perturbation of a beam propagating in the VBC fiber using an acoustic wave. The perturbation is caused by the modification of the refractive index of the fiber by the oscillating mechanical pressure of an acoustic wave. The period and strength of the acoustic wave are related to the acoustic wave frequency and amplitude, allowing dynamic control of the acoustic perturbation. Thus, a perturbation assembly 110 including AOT 2408 may be configured to vary the beam characteristics of a beam propagating in the fiber. In an example, piezo-electric transducer 2418 may create the acoustic wave and may be controlled by controller or driver 2420. The acoustic wave induced in AOT 2408 may be modulated to change and/or control the beam characteristics of the optical beam in VBC 200 in real-time. However, this is merely an example of a method for creating and controlling an AOT 2408 and claimed subject matter is not limited in this regard.

Thermal device 2410 may be used to induce perturbation of a beam propagating in VBC fiber using heat. The perturbation is caused by the modification of the RIP of the fiber induced by heat. Perturbation may be dynamically controlled by controlling an amount of heat transferred to the fiber and the length over which the heat is applied. Thus, a perturbation assembly 110 including thermal device 2410 may be configured to vary a range of beam characteristics. Thermal device 2410 may be controlled by controller 2450.

Piezo-electric transducer 2412 may be used to induce perturbation of a beam propagating in a VBC fiber using piezoelectric action. The perturbation is caused by the modification of the RIP of the fiber induced by a piezoelectric material attached to the fiber. The piezoelectric material in the form of a jacket around the bare fiber may apply tension or compression to the fiber, modifying its refractive index via the resulting changes in density. Perturbation may be dynamically controlled by controlling a voltage to the piezo-electric device 2412. Thus, a perturbation assembly 110 including piezo-electric transducer 2412 may be configured to vary the beam characteristics over a particular range.

In an example, piezo-electric transducer 2412 may be configured to displace VBC fiber 200 in a variety of directions (e.g., axially, radially, and/or laterally) depending on a variety of factors, including how the piezo-electric transducer 2412 is attached to VBC fiber 200, the direction of the polarization of the piezo-electric materials, the applied voltage, etc. Additionally, bending of VBC fiber 200 is possible using the piezo-electric transducer 2412. For example, driving a length of piezo-electric material having multiple segments comprising opposing electrodes can cause a piezoelectric transducer 2412 to bend in a lateral direction. Voltage applied to piezoelectric transducer 2412 by electrode 2424 may be controlled by controller 2422 to control displacement of VBC fiber 200. Displacement may be modulated to change and/or control the beam characteristics of the optical beam in VBC 200 in real-time. However, this is merely an example of a method of controlling displacement of a VBC fiber 200 using a piezo-electric transducer 2412 and claimed subject matter is not limited in this regard.

Gratings 2414 may be used to induce perturbation of a beam propagating in a VBC fiber 200. A grating 2414 can be written into a fiber by inscribing a periodic variation of the refractive index into the core. Gratings 2414 such as fiber Bragg gratings can operate as optical filters or as reflectors. A long-period grating can induce transitions among co-propagating fiber modes. The radiance, intensity profile, and/or divergence profile of a beam comprised of one or more modes can thus be adjusted using a long-period grating to couple one or more of the original modes to one or more different modes having different radiance and/or divergence profiles. Adjustment is achieved by varying the periodicity or amplitude of the refractive index grating. Methods such as varying the temperature, bend radius, and/or length (e.g., stretching) of the fiber Bragg grating can be used for such adjustment. VBC fiber 200 having gratings 2414 may be coupled to stage 2426. Stage 2426 may be configured to execute any of a variety of functions and may be controlled by controller 2428. For example, stage 2426 may be coupled to VBC fiber 200 with fasteners 2430 and may be configured to stretch and/or bend VBC fiber 200 using fasteners 2430 for leverage. Stage 2426 may have an embedded thermal device and may change the temperature of VBC fiber 200.

FIG. 25 illustrates an example process 2500 for adjusting and/or maintaining beam characteristics within a fiber without the use of free-space optics to adjust the beam characteristics. In block 2502, a first length of fiber and/or an optical beam are perturbed to adjust one or more optical beam characteristics. Process 2500 moves to block 2504, where the optical beam is launched into a second length of fiber. Process 2500 moves to block 2506, where the optical beam having the adjusted beam characteristics is propagated in the second length of fiber. Process 2500 moves to block 2508, where at least a portion of the one or more beam characteristics of the optical beam are maintained within one or more confinement regions of the second length of fiber. The first and second lengths of fiber may be comprised of the same fiber, or they may be different fibers.

Time Varying Beam Characteristics

Disclosed further herein are methods, systems, and apparatus operable to modulate one or more characteristics of an optical beam over a time the beam is conveyed through one or more fibers between an optical input and an optical output. Generally, perturbation of one or more lengths of fiber and/or a beam propagating within the one or more lengths of fiber is to change one or more characteristics of the beam conveyed to a predetermined destination (e.g., an output port of a process head). As described above, one or more lengths of fiber may be enlisted to vary one or more beam characteristics. A perturbation device within the optical system is to perturb the beam and/or length of fiber such that the beam's characteristics are altered in response to a control signal that may be generated open-loop, or with a closed-loop configuration that further relies on one or more response monitors. Characteristics of a beam (e.g., described elsewhere herein) varied within the VBC fiber may be controlled, for example, to time average the beam during a run-time phase of an optical system, and/or may be controlled for rapid configuration of an optical system during a setup or initialization phase.

Various ones of the above perturbation devices (e.g., piezo-electric transducers, certain translation or rotation stages) are well suited to rapid and/or continuous adjustments. Where the modulation of a beam characteristic is on a time scale small enough for the characteristic to vary during operation of an optical system, the time varying beam characteristic may be referred to as a “dynamic” beam characteristic. Through application of the structures and techniques described above, a fiber operable to impart variable beam characteristics (VBC) may be incorporated into an optical system to output a dynamic optical beam that may facilitate beam optimization and/or adjustment via rapid time averaging or sweeping of the beam characteristics within a time duration that the beam is being utilized in an application. A fiber-based modulator in accordance with embodiments may continuously vary a wider gamut of beam characteristics, and on a much shorter time scale, than is typical of an optical system employing a conventional system architecture that relies on free space optics and/or other techniques to define a few static system operating points.

Notably, the dynamic beam characteristics described herein are compatible with a wide variety of optical systems. For example, systems and methods described herein may be well suited to any of optical telecommunication systems, materials processing, chemical or physical sensing, and medical or biological applications. Also of note, systems described herein are compatible with a wide variety of lasers. For example, any of the lasers described elsewhere herein may be included in an optical system that further includes a modulator having one or more of the features described herein.

FIG. 29 is a flow diagram illustrating exemplary methods 2901 employing dynamic optical beam characteristics in accordance with some embodiments. Methods 2901 may be implemented, for example, by a fiber-based optical system, as described further below. Methods 2901 begin at block 2902 where an optical system is setup at operation 2902. System setup may entail any number of activities associated with configuring an optical system for a specific application and/or task prior to energizing an optical beam within the system. For example, in the context of materials processing, at block 2902 a laser power set point or target value may be defined based on a particular task (e.g., piercing or cutting in the context of materials processing). In further examples, an initial set point or target for one or more beam characteristics may also be defined at block 2902. The initial set point may be associated with a given intensity and/or divergence distribution desired for a beam and may be established by configuring a perturbation device into an initial state that is associated with the initial set point or target for one or more beam characteristics.

At block 2904, the optical source (e.g., fiber laser or fiber-coupled laser) is energized and a resultant optical beam is conveyed through the optical system, as initially setup at block 2902, to an external destination (e.g., a metal sheet in the context of materials processing). In some embodiments, methods 2901 continue at block 2906 where one or more process monitors collect process metric data. Block 2906 is illustrated in dashed line to emphasize that process monitoring is an optional input to block 2908. Any of the beam characteristics described above as being potentially variable may be varied from a prior state or value to a new state or value at block 2908. Furthermore, any of the techniques described above to vary a beam characteristic may be practiced at block 2908. For example, a perturbation device may be controlled to act upon one or more lengths of fiber at block 2908 to vary one or more beam characteristics, such as, but not limited to, intensity or divergence distribution. Notably, such action is performed while the optical source remains energized, and therefore block 2908 dynamically adjusts one or more beam characteristics. In some embodiments, the variation at block 2908 entails a sweeping or oscillating of a beam characteristic through values of the characteristic. Such a sweep or oscillation may be according to any predetermined stepped or continuous function of time. For example, a beam characteristic may be modulated according to a sinusoidal function having some predetermined amplitude and frequency. The sweeping function may then dictate time averaging of the beam characteristic over the duration of time that methods 2901 are performed. Time averaging may advantageously smooth a beam characteristic. For example, optical propagation modes within one or more lengths of fiber may be time averaged (i.e. modal averaging) by modulating the spatial profile of a beam according to one or more of the methods described above (e.g., in the context of FIG. 4 through FIG. 10).

At block 2908 a fiber perturbation device may act upon one or more lengths of fiber, and/or upon a beam within the length(s) of fiber, to continuously vary beam characteristics in an open-loop control mode. Alternatively, methods 2901 may entail closed-loop control of a fiber perturbation device that is to continuously vary beam characteristics at block 2908 in some manner that is further based on a feedback signal indicative of the process metric data collected at block 2906. Hence, for embodiments where process monitoring is performed at block 2906, blocks 2906 and 2908 may be concurrently implemented with the latter dependent on the former at least in some capacity. Methods 2901 continue at block 2910 where the beam source (e.g., fiber laser or fiber-coupled laser) is de-energized and the optical beam terminated, for example upon completion of the task for which the system was initially setup. As denoted by the dashed line between blocks 2910 and 2902, methods 2901 may be repeated any number of times during a lifetime of an optical system.

FIG. 30 illustrates an exemplary optical beam delivery system 3001 suitable for time varying optical beam characteristics during materials processing. Beam delivery device 3001 may implement methods 2901, for example. Notably, free-space optics are not required by system 3001 to implement methods 2901. In beam delivery device 3001, components having the same reference number introduced in the context of laser system 2200 (FIG. 22A-22B) may have substantially the same function and/or may have any of the features or attributes described above. In the example illustrated in FIG. 30, beam delivery system 3001 includes VBC fiber assembly 2202, which may be configured to provide variable beam characteristics according to any of the examples described elsewhere herein. In some embodiments, VBC fiber assembly 2202 comprises first length of fiber 104, second length of fiber 108, and a perturbation device 110, for example substantially as introduced in FIGS. 1 and 2. As further shown in FIG. 30, VBC fiber assembly 2202 is located between VBC delivery fiber 2240 and fiber 2212, which may receive optical beam 2210 as output by a laser source 3040. VBC delivery fiber 2240 may comprise second length of fiber 108 or an extension of second length of fiber 108 that modifies, maintains, and/or confines adjusted beam characteristics. Beam 2210 is coupled into VBC fiber assembly 2202 via fiber 2212. Fiber assembly 2202 may have any optical input to receive beam 2210, such as, but not limited to, a fiber splice junction to feeding fiber 2212, a free air gap, a free-space or bulk optic, glue, index-matching material, a butt-couple, or the like, or any combination thereof. VBC fiber assembly 2202 is to vary the characteristics of beam 2210 as it is conveyed through one or more length of fiber within fiber assembly 2202, in accordance with one or more of the various examples described above.

As shown in FIG. 30, fiber assembly 2202 is communicatively coupled to a controller 3030. Controller 3030 is to output a control signal or drive signal that dictates actions of a perturbation device within VBC fiber assembly 2202. The resulting output of fiber assembly 2202 is a dynamically adjusted beam 3014 that has one or more time-varying beam characteristics. Dynamically adjusted beam 3014 is coupled through VBC delivery fiber 2240. VBC delivery fiber 2240 delivers dynamically adjusted beam 3014 to any suitable destination through any suitable optical path. In some examples, the optical path may include a free-space optics assembly, such as, but not limited to free-space optics assembly 2208 (e.g., as illustrated in FIG. 22), or an alternative fiber-to-fiber coupler (FFC). In other embodiments, no FFC is present and dynamically adjusted beam 3014 is conveyed directly to process head 2206. In still other embodiments, dynamically adjusted beam 3014 is conveyed from delivery fiber 2240 to process head 2206 by one or more process fibers 2204. If process fiber 2204 is present, the RIP of process fiber 2204 may preserve dynamically adjusted beam 3014. Process head 2206 may have any attributes known to be suitable for a given application, such as any of those features described above (e.g., guided wave optics, free-space optics such as lenses, mirrors, optical filters, diffraction gratings), beam scan assemblies such as galvanometer scanners, polygonal mirror scanners, or other scanning systems that are used deliver the beam to a destination 3190. In exemplary materials processing applications, external destination 3190 is any suitable workpiece, such as sheet metal, another sheet good, tubing, shaped material, etc.

Fiber assembly 2202 may include one or more lengths of fiber between an optical input and an optical output of the assembly. In some examples, where a first length of fiber is optically coupled to the optical input, the first length of fiber may be fused to fiber 2212. A second length of fiber of fiber assembly 2202 may comprise a confinement region that is optically coupled to an output of fiber assembly 2202. Any of a fiber splice junction, a free air gap, a free-space or bulk optic, butt-couple, glue, index-matching material, or the like, or any combination thereof may optically couple an output of fiber assembly 2202 to VBC delivery fiber 2240. Fiber assembly 2202 may include any perturbation device (e.g., having one or more of the features and/or attributes described above) suitable for modulating optical beam 2210 into dynamically adjusted beam 3014. In some exemplary embodiments, the action(s) include one or more of the perturbations described above in the context of VBC fiber, such one or more physical actions on one or more fiber lengths to modulate beam intensity and/or divergence distributions. In the context of fiber assembly 2202, a perturbation device is to modulate, through action(s) upon a first fiber length 3004 and/or a second length 3008 within fiber assembly 2202, characteristics of the beam conveyed through at least confinement region of the second fiber length. In some exemplary embodiments, the action(s) include one or more of the perturbations described above in the context of VBC fiber. In some examples, the action(s) comprise one or more physical actions on either or both of the first and second fiber lengths. Alternatively, a perturbation device may act directly on input beam as it is propagated through fiber lengths within fiber assembly 2202 in a manner that modulates the beam characteristics. As noted above in the context of the two-dimensional beam intensity distributions shown in FIG. 5-10, the spatial profile of a beam may be varied by modulating a fiber bend radius, fiber bend length, and/or various other parameters using the perturbation device. Hence, in FIG. 30 a spatial profile of dynamically adjusted beam 3014 may be continuously varied over time according to a VBC control/drive signal 3024 output from controller 3030.

In the illustrated example, controller 3030 is external of fiber assembly 2202. Controller 3030 may alternatively be a component of fiber assembly 2202 and/or one or more perturbation device responsible for acting upon fibers of fiber assembly 2202 may also be a component of another assembly that further includes controller 3030. In some embodiments, controller 3030 is a function generator, which may output any time-varying analog waveform that may be predefined according to one or more of a frequency, amplitude, and phase. In some examples, controller 3030 comprises a driver and drive/control signal 3024 is suitable for directly driving a perturbation device into different states that act upon a fiber to varying degrees. In some other embodiments, controller 3030 outputs a time-varying digital signal having at least two logic levels (e.g., HI/LO), and a perturbation device is to enter different states that act upon a fiber to varying degrees based one of more of the logic levels conveyed through drive/control signal 3024. Controller 3030 may therefore comprise a programmable logic processor.

Optionally, controller 3030 may also output an additional control/drive signal (not depicted) to a free-space optics assembly functioning as a FFC downstream of fiber assembly 2202. Such an additional control/drive signal may be a supplement to, or an alternative to, VBC control signal 3024. However, an additional control/drive signal may be absent, for example, where there is no ability to modulate any components of a FFC. For some embodiments where an additional control/drive signal is present, components of the FCC may be adjusted based on the additional control/drive signal on a time scale significantly longer than VBC modulation responsive to VBC control/drive signal 3024. In some examples, modulations of components of a free-space optics assembly (e.g., assembly 2208 illustrated in FIG. 22) are triggered by an additional control/drive signal while laser source 3040 is de-energized.

FIG. 31 is a timing diagram illustrating operation of an optical beam delivery system having time-varying optical beam characteristics in accordance with some embodiments. As shown, a system clock CLK is operative at a reference frequency. Controller 3030 (FIG. 30) may implement system clock CLK, for example. Beginning at an initial time t₀, VBC modulation 3105 is performed, for example during an initial setup or configuration of beam characteristics. VBC modulation 3105 may be performed during block 2902 in methods 2901 (FIG. 29). At a later time t₁ (FIG. 31), laser output 3110 begins, initiating a run-time phase of system operation. VBC modulation may then begin upon energizing a beam source. In the example of FIG. 31, during a subsequent clock cycle, VBC modulation 3115 is initiated at time t₂. VBC modulation 3115 may be performed at block 2908 of methods 2901 (FIG. 29), for example. VBC modulation 3114 may continue for a predetermined duration (e.g., until some later time t₃) or may be terminated in response to termination of laser output 3110 at time t₄. Another iteration of system initialization and operation may then proceed in substantially the same manner thereafter. Alternatively, VBC modulation may be initiated prior to energizing a source laser (e.g., as for VBC modulation 3105) and continued while the source laser is energized (e.g., merging VBC modulation 3105 with VBC modulation 3115).

In some embodiments, an optical beam delivery system suitable for delivering a beam having dynamic beam characteristics further includes one or more monitors, which may output a feedback signal that is enlisted in the operation of the beam delivery system. Any monitor known to be suitable for providing operational feedback to an optical system may be included in systems configured for delivering a beam having dynamic beam characteristics. While such a feedback signal may be employed for any purpose, in some examples dynamic beam characteristics are controlled based, at least in part, on a feedback signal output by a monitor. FIG. 32 illustrates a schematic of an optical beam delivery system 3201 that includes a process monitor 3210. Process monitor 3210 outputs a feedback signal 3242, which is input into controller 3030. In beam delivery device 3201, components having the same reference number introduced in the context of laser system 3001 (FIG. 30) may have substantially the same function and/or may have any of the features or attributes described above. Hence, beam delivery system 3201 is operable to provide the dynamically adjusted beam 3014 substantially in the same manner as beam delivery system 3001. In addition, process monitor 3210 is to detect a response that is indicative of operation of the delivery system 3210, and controller 3030 is to control a perturbation device to dynamically vary the one or more beam characteristics based on feedback signal 3242.

For the embodiment illustrated in FIG. 32, beam delivery system 3201 is illustrated in the context of a materials processing application. Process monitor 3210 is to detect a response (e.g., external of delivery system 3201) that is indicative of some process quality metric associated with a given processing task (e.g., piercing, cutting, welding, heating, material deposition, etc.). Process monitor 3210 may be any sensor capable of detecting a response known to be suitable for generating an output signal that has a correlation with a process (quality) metric. Process monitor 3210 may be any monitor known to be suitable for automating an optical system for a given material processing task. For example, in some embodiments, process monitor 3210 comprises an acoustic monitor (e.g., including one or more microphones). In some other embodiments, process monitor 3210 comprises an optical sensor (e.g., including one or more photodiodes, a CCD or CMOS camera, a spectrometer or other spectrum analyzer, or any combination thereof). In still other embodiments, process monitor 3210 comprises a pyrometer or other thermal sensor. Feedback signal 3242 is to be employed for controlling dynamic variation of one or more of the variable beam characteristics while the beam is being conveyed to a workpiece. System 3201 is therefore suitable for performing methods 2901 (FIG. 29), where block 2906 is to be implemented by process monitor 3210.

FIG. 33 is a timing diagram illustrating operation of an optical beam delivery system having variable optical beam characteristics, in accordance with some embodiments. The illustrated timing diagram may represent operation of system 3201 (FIG. 32) during performance of methods 2901 (FIG. 29), for example. As shown in FIG. 33, a system clock CLK is operative at a reference frequency. System clock CLK may be maintained by the controller 3030 (FIG. 30), for example. Beginning at an initial time t₀, VBC modulation 3105 is performed, for example during an initial setup or configuration of beam characteristics. VBC modulation 3105 is performed prior to energizing a source laser, for example during a setup phase of system operation. At a later time t₁, laser output 3110 begins, demarking the beginning of a run-time phase of operation. Monitor feedback 3305 may then begin at some subsequent time t₂. VBC modulation 3115 may then be triggered upon monitor feedback 3305 satisfying one or more predetermined criteria. In the example of FIG. 33, VBC modulation 3115 is initiated at time t₃ and continues for some time increment until being halted at time t₄, for example in further response to monitor feedback 3305. While still in the run-time phase of operation, VBC modulation 3115 may again be triggered at time t₅, for example upon monitor feedback 3305 satisfying one or more predetermined criteria, until being halted at time t₆. Subsequently, at time t₇, laser output 3110 is discontinued along with monitor feedback 3305. Another iteration of system initialization and operation may then proceed in substantially the same manner thereafter.

In some embodiments, a perturbation device is to vary one or more beam characteristics continuously about a target value while a system laser is energized. Continuous variation may permit time averaging and/or smoothing of one or more beam characteristics. Continuous variation may also be useful for optimizing and/or stabilizing a process metric. For example, a beam characteristic that is varied about according to a known function may induce a correlated function within a feedback signal that may have significant levels of noise. Lock-in detection techniques may then improve the signal-to-noise ratio of the feedback signal so that the target of the beam characteristic may be more readily adjusted or updated based on the detected response. Therefore, beam characteristics may be dithered or modulated about a set point/target according to a predetermined function (e.g., in an open-loop manner), and beam characteristics may be further adjusted to different set points/targets based on a feedback signal (e.g., closed-loop) to improve process control. FIG. 33 further illustrates continuous modulation 3315 initiated at time t₂ that continues until laser output ceases at time t₇. VBC modulation 3115 is then triggered at specific times as described above, for example to move a center point of the VBC modulation 3315 upon monitor feedback 3305 satisfying one or more predetermined criteria.

FIG. 34 illustrates a graph of an exemplary process metric and a variable beam characteristic as a function of process time, in accordance with some embodiments. In FIG. 34, a variable beam characteristic is plotted to the left axis as a function of time. The VBC may be any of those described above, such as, but not limited to, beam diameter, spatial profile, divergence distribution, beam parameter product, intensity distribution, luminance, optical intensity, power density, radiance, or radial beam position. As one example, the VBC values on the left axis may represent a modulated intensity or divergence profile along one axis. Notably, VBC values may be modulated along multiple axes and by different amounts (i.e., modulation need not be symmetric about a fiber axis). A process metric 3410 detected by a suitable process monitor is plotted as a function of time to the right axis. Process metric 3410 may be indicative of cutting quality (e.g., speed or edge quality), piercing (e.g., hole size and taper, speed), keyhole stability, spatter reduction, or porosity reduction gap bridging (for welding applications). For other applications outside of materials processing, process metric 3410 may be indicative of speckle (e.g., for illumination and/or sensing applications).

As shown, at an initial time t₀, VBC 3402 has a periodic (e.g., sinusoidal) modulation with a predetermined amplitude 3404 and a predetermined frequency. In other embodiments, the temporal modulation profile of VBC 3402 may instead have other shapes (e.g., triangular, saw tooth, etc.). Initially, from time t₀ to time t₁, VBC 3402 is modulated about a target 3401. During the time increment t₀-t₁, process metric 3410 may be processed, for example with a lock-in amplifier that may be a component of a perturbation controller. The lock-in amplifier may compare the feedback signal with the sinusoidal VBC modulation signal as a reference. From the referenced modulation signal, the lock-in amplifier (or other filter) may detect a response signal 3415 (illustrated in FIG. 34 as a white dotted line) within process metric 3410. Response signal 3415 may have a discernable amplitude, and a phase relative to the referenced sinusoidal VBC modulation signal. Upon acquisition of response signal 3415, a target or center point of VBC 3402 may be adjusted based on response signal 3415. In the illustrated example, at time t₁, process metric 3410 deviates (e.g., as a result of a non-uniformity in a workpiece being processed, etc.). In response, VBC 3402 is adjusted to a new target 3405 with the periodic (e.g., sinusoidal) VBC modulation continuing. Upon detecting another deviation in process metric 3410 at time t₂, VBC 3402 is again adjusted to a new target 3407. The control effort based on process metric 3410 may thereby improve process stability. Any number of adjustments in the variable beam characteristic may be performed throughout the run-time (e.g., at times t₃ and t₄) to compensate for drift or other variations detected in process metric 3410 as a processing task is performed.

FIG. 35 schematic of an exemplary networked computer system 3500, which may be configured as a controller, to read one or computer readable media and to execute one or more of the methods described herein. System 3500 may be an embedded device although system 3500 is not limited to this context. For example, system 3500 may be incorporated into a laptop computer, ultra-laptop computer, tablet, touch pad, portable computer, handheld computer, palmtop computer, and so forth.

System 3500 includes a device platform 3502 that may implement all or a subset of the various dynamic VBC control methods described above. CPU 3550 may include logic circuitry to generate a perturbation drive signal for controlling a perturbation device, for example through A/D converter 3355. In some embodiments, one or more computer readable media may store instructions, which when executed by CPU 3550, cause the processor to generate a drive signal for controlling a time varying beam characteristic. One or more monitor signals may be received into system 3555, for example through microphone/camera interface 3513. The monitor signals may be stored in memory 3512.

In some embodiments, device platform 3502 is coupled to a human interface device (HID) 3520. HID 3520 may include any television type monitor or display coupled to platform 3502 via radio 3518 and/or network 3560. HID 3520 may include, for example, a computer display screen, touch screen display, video monitor, television-like device, and/or a television. Platform 3502 may collect monitor data, which may be processed by CPU 3550 programmed to execute a lock-in detection algorithm. Alternatively, platform 3505 further comprises a lock-in detection amplifier circuit, which may be one ASIC of chipset 3505. A navigation controller 3552 including one or more navigation features may be used to interact with, for example, device platform 3502 and/or HID 3520.

Under the control of one or more software applications 3516, device platform 3502 may display user interface 3522 on HID 3520. Movements of the navigation features of controller 3552 may be replicated on a display (e.g., HID 3520) by movements of a pointer, cursor, focus ring, or other visual indicators displayed on the display. For example, under the control of software applications 3516, the navigation features located on navigation controller 3550 may be mapped to virtual navigation features displayed on user interface 3522.

Device platform 3502 may include any combination of microphone/camera interface 3513, chipset 3505, processor 3550, controller 3552, memory 3512, storage 3511, applications 3516, and radio 3518 known in the art. Chipset 3505 may provide intercommunication among processor 3550, controller 3552, memory 3512, storage 3511, applications 3516, A/D converter 3555 and radio 3518.

Processor 3550 may be implemented as one or more Complex Instruction Set Computer (CISC) or Reduced Instruction Set Computer (RISC) processors; x86 instruction set compatible processors, multi-core, or any other microprocessor or central processing unit (CPU). Memory 3512 may be implemented as a volatile memory device such as, but not limited to, a Random Access Memory (RAM), Dynamic Random Access Memory (DRAM), or Static RAM (SRAM). Storage 3511 may be implemented as a non-volatile storage device such as, but not limited to, a magnetic disk drive, optical disk drive, tape drive, an internal storage device, an attached storage device, flash memory, battery backed-up SDRAM (synchronous DRAM), and/or a network accessible storage device. Radio 3518 may include one or more radios capable of transmitting and receiving signals using various suitable wireless communications techniques. Such techniques may involve communications across one or more wireless networks. Example wireless networks include (but are not limited to) wireless local area networks (WLANs), wireless personal area networks (WPANs), wireless metropolitan area network (WMANs), cellular networks, and satellite networks. In communicating across such networks, radio 3518 may operate in accordance with one or more applicable wireless standards versions.

In embodiments, system 3500 may be implemented as a wireless system, a wired system, or a combination of both. When implemented as a wireless system, system 3500 may include components and interfaces suitable for communicating over a wireless shared media, such as one or more antennas, transmitters, receivers, transceivers, amplifiers, filters, control logic, and so forth. An example of wireless shared media may include portions of a wireless spectrum, such as the RF spectrum and so forth. When implemented as a wired system, system 3500 may include components and interfaces suitable for communicating over wired communications media, such as input/output (I/O) adapters, physical connectors to connect the I/O adapter with a corresponding wired communications medium, a network interface card (NIC), disc controller, video controller, audio controller, and the like. Examples of wired communications media may include a wire, cable, metal leads, printed circuit board (PCB), backplane, switch fabric, semiconductor material, twisted-pair wire, co-axial cable, fiber optics, and so forth.

Embodiments described herein may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements or modules include: processors, microprocessors, circuitry, circuit elements (e.g., transistors, resistors, capacitors, inductors, and so forth), integrated circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), logic gates, registers, semiconductor device, chips, microchips, chip sets, and so forth. Examples of software elements or modules include: applications, computer programs, application programs, system programs, machine programs, operating system software, middleware, firmware, routines, subroutines, functions, methods, procedures, software interfaces, application programming interfaces (API), instruction sets, computing code, computer code, code segments, computer code segments, data words, values, symbols, or any combination thereof. Determining whether an embodiment is implemented using hardware elements and/or software elements may vary in accordance with any number of factors considered for the choice of design, such as, but not limited to: desired computational rate, power levels, heat tolerances, processing cycle budget, input data rates, output data rates, memory resources, data bus speeds and other design or performance constraints.

One or more aspects of at least one embodiment may be implemented by representative instructions stored on a machine-readable storage medium. Such instructions may reside, completely or at least partially, within a main memory and/or within a processor during execution thereof by the machine, the main memory and the processor portions storing the instructions then also constituting a machine-readable storage media. Programmable logic circuitry may have registers, state machines, etc. configured by the processor implementing the computer readable media. Such logic circuitry, as programmed, may then be understood to have been physically transformed into a system falling within the scope of the embodiments described herein. Instructions representing various logic within the processor, which when read by a machine may also cause the machine to fabricate logic adhering to the architectures described herein and/or to perform the techniques described herein. Such representations, known as cell designs, or IP cores, may be stored on a tangible, machine-readable medium.

While certain features set forth herein have been described with reference to various implementations, this description is not intended to be construed in a limiting sense. Hence, various modifications of the implementations described herein, as well as other implementations, which are apparent to persons skilled in the art to which the present disclosure pertains are deemed to lie within the spirit and scope of the present disclosure. It will be recognized that the invention is not limited to the embodiments so described, but can be practiced with modification and alteration without departing from the scope of the appended claims. The above embodiments may include the undertaking of only a subset of such features, undertaking a different order of such features, undertaking a different combination of such features, and/or undertaking additional features than those features explicitly listed. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Having described and illustrated both general and specific principles of examples of the presently disclosed technology, it should be apparent that the examples may be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims. 

1. An optical beam delivery system, comprising: an optical input to receive an optical beam; one or more lengths of fiber between the optical input and an optical output, wherein at least one of the lengths of fiber comprises a confinement region that is optically coupled to the output; a perturbation device to modify, through action upon the one or more lengths of fiber, one or more beam characteristics of the optical beam conveyed by the confinement region to the output; and a controller coupled to the perturbation device, wherein the controller is to control the perturbation device to vary the one or more beam characteristics over a period of time that the optical beam is received.
 2. The beam delivery system, of claim 1, further comprising a laser coupled to the optical input, the laser to generate the optical beam when energized, and wherein the controller is to control the perturbation device to vary the one or more beam characteristics continuously for at least some portion of time while the laser is energized.
 3. The beam delivery system of claim 2, wherein the controller is to vary the one or more beam characteristics according to a sinusoidal modulation signal having a predetermined amplitude and a predetermined frequency.
 4. The beam delivery system of claim 1, further comprising a process monitor to output a feedback signal to the controller, wherein the process monitor is to detect a response external of the delivery system that is indicative of a process metric, and wherein the controller is to vary the one or more beam characteristics based on the feedback signal.
 5. The beam delivery system of claim 4, wherein the process monitor comprises at least one of an acoustic sensor, an optical sensor, or a thermal sensor.
 6. The beam delivery system of claim 4, wherein: the controller is to control the perturbation device to vary the one or more beam characteristics continuously about a beam characteristic target while the laser is energized; and the controller is to control the perturbation device to move the beam characteristic target based on the feedback signal.
 7. The beam delivery system of claim 6, wherein the controller is to: vary the one or more beam characteristics to have sinusoidal modulation with a predetermined amplitude and a predetermined frequency; and move the beam characteristic target by determining a correlation between the feedback signal and the modulation signal.
 8. The beam delivery system of claim 6, wherein the controller comprises a lock-in amplifier to compare the feedback signal with the modulation signal.
 9. The beam delivery system of claim 1, wherein the one or more beam characteristics comprise beam diameter, spatial profile, divergence distribution, beam parameter product, intensity distribution, luminance, optical intensity, power density, radiance, radial beam position, or any combination thereof.
 10. The beam delivery system of claim 1, wherein the action upon the one or more lengths of fiber alters a coupling of the optical beam between a first length of the fiber and a confinement region of a second length of fiber.
 11. The beam delivery system of claim 10, wherein: the one or more lengths of fiber further comprise a first length of fiber end-coupled with a second length of fiber; and the first length of fiber has a first refractive index profile (RIP) and the second length of fiber has a second RIP.
 12. The beam delivery system of claim 10, wherein: the first length of fiber or the second length of fiber or a combination thereof includes at least one divergence structure configured to modify a divergence profile of the optical beam and wherein the divergence structure comprises a first material having a lower index of refraction than a second material encompassing the divergence structure.
 13. A laser system, comprising: a laser to generate an optical beam when energized; an optical input coupled to the laser to receive the optical beam; one or more lengths of fiber between the optical input and an optical output, wherein at least one of the lengths of fiber comprises a confinement region that is optically coupled to the output; a perturbation device to modify, through action upon the one or more lengths of fiber, one or more beam characteristics of the optical beam conveyed by the confinement region to the output; and a controller coupled to the perturbation device, wherein the controller is to control the perturbation device to vary the one or more beam characteristics over a period of time that the laser is energized.
 14. The laser system of claim 13, further comprising: a process monitor to output a feedback signal to the controller; and wherein the controller comprises a lock-in amplifier to compare the feedback signal with the modulation signal; and wherein the controller is to vary the one or more beam characteristics according to a periodic modulation signal having a predetermined amplitude and a predetermined frequency, and move the beam characteristic target based on the comparison between the feedback signal and the modulation signal.
 15. An optical beam delivery method, the method comprising: receiving an optical beam into one or more lengths of fiber, wherein at least one of the lengths of fiber comprises a confinement region that is optically coupled to an optical output; and varying one or more beam characteristics of the optical beam through action upon the one or more lengths of fiber over a period of time of the receiving.
 16. The method of claim 15, further comprising: energizing a laser to generate the optical beam; and varying the one or more beam characteristics while the laser is energized.
 17. The method of claim 16, further comprising: collecting process metric data; generating a feedback signal indicative of the process metric data; and varying the one or more beam characteristics based on the feedback signal.
 18. The method of claim 17, wherein the one or more beam characteristics are varied continuously about a beam characteristic target, and the beam characteristic target is updated based on the feedback signal.
 19. The method of claim 18, wherein varying the one or more beam characteristics further comprises varying the one or more beam characteristics according to a sinusoidal modulation signal having a predetermined amplitude and a predetermined frequency.
 20. The method of claim 19, further comprising determining a correlation between the feedback signal and the modulation signal; and updating the beam characteristic target based on the correlation.
 21. The method of claim 20, wherein determining the correlation further comprises comparing the feedback signal with the modulation signal. 